THE CATERPILLAR. 57 



the flea, and the spider, change their covering 

 from the same necessity ; and growing too large 

 for the crust in which they have been for some 

 time enclosed, burst it for another. This pe- 

 riod is probably that of their growth ; for as soon 

 as their new skin is hardened round them, the 

 animal's growth is necessarily circumscribed while 

 it remains within it. With respect to caterpil- 

 lars, many of them change their skins five or 

 six times in a season ; and this covering, when 

 cast off, often seems so complete, that many might 

 mistake the empty skin for the real insect. Among 

 the hairy caterpillars, for instance, the cast skin 

 is covered with hair ; the feet, as well gristly as 

 membraneous, remain fixed to it ; even the parts 

 which nothing but a microscope can discover are 

 visible in it : in short, all the parts of the head, 

 not only the skull, but the teeth. 



In proportion as the time approaches in which 

 the caterpillar is to cast its old skin, its colours 

 become more feeble, the skin seems to wither and 

 grow dry, and in some measure resembles a leaf 

 when it is no longer supplied with moisture from 

 the stock. At that time the insect begins to find 

 itself under a necessity of changing ; and it is not 

 effected without violent labour, and perhaps pain. 

 A day or two before the critical hour approaches, 

 the insect ceases to eat, loses its usual activity, 

 and seems to rest immoveable. It seeks some 

 place to remain in security ; and no longer timo- 

 rous, seems regardless even of the touch. It is 

 now and then seen to bend itself and elevate its 

 back j again it stretches to its utmost extent : it 



